Concepts for Chemical Degradation of Materials and Structures
|Bearbeitung:||Dipl.-Ing. Milena Möhle, Prof. Dr.-Ing. Udo Nackenhorst|
The degradation of materials is of great importance especially when they are expected for a long service time or when it is not easily accessible for maintenance, for example like natural gas pipelines.
Today, it is a major political aim to replace the short running natural energy sources by renewable energy ones, like wind turbines or solar panels. Due to the irregular character of most of those energy sources, the energy conversion in turn fluctuates. To provide energy for the market with comparatively static demand, one idea is to store energy in form of hydrogen gas. This gas could be distributed through the existing gas pipeline system, either in addition to or, on the longer term, even as a successor to the current natural gas. However, the effect of hydrogen induced embrittlement within the pipeline system is not well understood yet.
In order to describe the mechanism of hydrogen embrittlement, two factors need to be considered. On the one hand the hydrogen distribution within the material has to be determined and on the other hand the mechanical response of the material to the hydrogen has to be predicted.
The hydrogen distribution within the lattice is strongly affected by the stress state within the material. In regions with high hydrostatic stresses, hydrogen accumulates. Furthermore, the trapping of hydrogen within dislocations or at grain boundaries has an impact on the hydrogen distribution. Thus, an accumulation of hydrogen is expected in regions of high plastic deformation. In order to capture steep gradients of the hydrogen concentration at for example a crack tip, a combination of the Galerkin method in space and the Time Discontinuous Galerkin (TDG) method is applied. Once the hydrogen concentration is determined, the effect on the material properties needs to be calculated. Within this project, it is assumed that the hydrogen enhanced localized plasticity (HELP) mechanism is the major effect for failure. This mechanism is characterized by high concentrations of hydrogen either shielding dislocations from each other or decreasing the interaction of dislocations, leading to a local enhancement of plasticity. This localized softening appears as brittle behavior on macro scale. Therefore, the yield stress is defined as a function of the total hydrogen concentration (trapped and lattice hydrogen).
The aim of this project is to describe the effect of hydrogen embrittlement to get a first estimate on possible effects on the service conditions of pipelines. Therefore a worst case scenario is assumed: A critical crack within the pipeline is investigated with and without hydrogen, in order to quantify the impact of hydrogen on the structure.